Varistor temperature coefficient testing measures how the varistor voltage and leakage current of metal oxide varistors change over a specified temperature range, a critical parameter for verifying surge protection performance under real-world thermal conditions. This testing is essential for component manufacturers, power supply designers, and quality engineers who must demonstrate compliance with international standards such as IEC 61051 and GB/T 10193. In this article, we explain the technical principles, sample requirements, test procedures, calculation methods, and typical acceptance criteria for varistor temperature coefficient testing performed by accredited third-party laboratories.
Technical Overview
A varistor, also known as a voltage-dependent resistor (VDR), is a ceramic semiconductor component—most commonly based on zinc oxide (ZnO)—whose resistance decreases sharply when the applied voltage exceeds a threshold known as the varistor voltage. Because the nonlinear conduction mechanism of ZnO grain boundaries is inherently temperature-sensitive, the electrical characteristics of a varistor drift as ambient temperature changes. Varistor temperature coefficient testing quantifies this drift and confirms that the component remains within specified limits across its operating temperature range.
The temperature coefficient of varistor voltage is defined as the relative change in varistor voltage per degree of temperature change, typically expressed in percent per degree Celsius (%/°C). It is calculated using the measured varistor voltage at two different temperatures according to the following relationship:
αT = [(UT2 − UT1) / (UT1 × (T2 − T1))] × 100%
where UT1 is the varistor voltage measured at reference temperature T1 (usually 20°C or 25°C), and UT2 is the varistor voltage measured at the category temperature T2 (for example, −40°C or +85°C). For zinc oxide varistors, the coefficient is generally negative, meaning the varistor voltage decreases as temperature increases. High-quality MOVs typically exhibit an absolute temperature coefficient of no more than 0.1%/°C across the standard category temperature range, although the exact limit is always defined by the relevant detail specification.
In addition to the varistor voltage coefficient, testing frequently evaluates the temperature dependence of leakage current, since leakage under the maximum continuous operating voltage (MCOV) can increase significantly at elevated temperatures and affect long-term reliability, self-heating, and thermal stability in the end application.
Testing Samples
Disc-Type Radial Lead Varistors
Radial-leaded epoxy-coated disc varistors are the most common sample type. These components, ranging roughly from 5 mm to 20 mm in diameter, are widely used in AC power inputs, power strips, and industrial surge protection. Samples should be representative of normal production and must be handled without stressing the ceramic body or leads.
Surface-Mount (SMD) Chip Varistors
Multilayer chip varistors used on printed circuit boards are tested for temperature coefficient to verify performance in consumer electronics, communication modules, and portable devices. Due to their lower varistor voltage ratings, measurement currents and fixture parasitics require special attention during testing.
High-Energy Block and Strap Varistors
Large block-type MOVs used in surge protective devices (SPDs), power distribution, and railway systems are evaluated over their full category temperature range. Their larger thermal mass requires longer temperature stabilization times before electrical measurement.
Automotive-Grade Varistors
Varistors intended for vehicle electronics are typically tested over an extended temperature range, for example from −40°C up to +125°C or higher, consistent with automotive component qualification frameworks such as AEC-Q200. These samples must demonstrate stable temperature characteristics under severe thermal environments.
Testing Items
A comprehensive varistor temperature coefficient test program typically includes the following measurement items:
- Varistor voltage (V1mA or V0.1mA) measured at reference temperature (20°C or 25°C)
- Varistor voltage measured at the lower category temperature (for example, −40°C or −25°C)
- Varistor voltage measured at the upper category temperature (for example, +85°C, +105°C, or +125°C)
- Calculated temperature coefficient of varistor voltage in %/°C
- Leakage current at a specified DC voltage (commonly 75% of the varistor voltage or at MCOV) measured at high temperature
- Voltage change rate (percentage deviation from the reference value) after thermal exposure
- Visual examination for coating cracks, delamination, or marking degradation after temperature conditioning
Testing Methods
Reference Measurement
The test begins by stabilizing the samples at the reference ambient temperature of 20°C ± 2°C (or 25°C ± 2°C where specified). A regulated DC current—typically 1 mA for disc varistors or 0.1 mA for small chip types—is applied for a short pulse duration, usually no more than a few hundred milliseconds to avoid self-heating, and the resulting varistor voltage is recorded. This value serves as the baseline for all subsequent calculations.
Low-Temperature and High-Temperature Measurements
The samples are then placed in a climatic test chamber and exposed sequentially to the specified lower and upper category temperatures. At each temperature point, the samples are allowed to soak until thermal equilibrium is reached, commonly 30 minutes to several hours depending on sample mass. The varistor voltage is then remeasured at the same test current, with measurements taken while the devices remain at temperature. Pulse duration and measurement timing must be tightly controlled so that joule heating does not distort the readings.
Calculation and Acceptance Criteria
Using the recorded voltage values, the temperature coefficient is calculated between the reference temperature and each extreme temperature. The result is compared against the limit specified in the applicable detail specification. A typical acceptance criterion for general-purpose ZnO varistors is an absolute temperature coefficient not exceeding 0.1%/°C, while automotive and industrial-grade products may be subject to stricter limits. If the leakage current test is included, the sample is biased at the specified DC voltage at high temperature and the steady-state current is compared with the specified maximum.
Equipment and Conditions
Reliable results depend on calibrated instrumentation and controlled environmental conditions:
- Temperature and humidity chamber capable of maintaining the specified category temperatures within ±2°C
- Regulated DC voltage and current source with programmable pulse duration for varistor voltage measurement
- High-impedance electrometer or picoammeter for leakage current measurement
- Low-thermal-EMF test fixtures with guarded connections to minimize parasitic leakage paths
- Condensation prevention measures, such as dry nitrogen purging, when moving samples between temperature extremes
Applicable Standards
Varistor temperature coefficient testing is generally performed in accordance with the following international and national standards:
- IEC 61051-1 – Varistors for use in electronic equipment: Generic specification
- IEC 61051-2 – Sectional specification for surge suppression varistors
- GB/T 10193 – Generic specification for varistors for use in electronic equipment (Chinese national standard aligned with IEC 61051)
- UL 1449 – Surge protective devices, covering thermal and endurance considerations for MOV-based products
- JIS C 5381 series – Japanese standards for surge protective components incorporating varistors
Applications
Temperature coefficient test data directly supports design and qualification decisions across a wide range of industries:
- AC and DC power supplies, where varistors on the mains input must tolerate ambient and internal temperature rise while maintaining clamping performance
- Surge protective devices (SPDs) for buildings, telecommunications, and photovoltaic systems, where long-term thermal stability is safety-critical
- Automotive electronics, including on-board chargers, infotainment, and body control modules exposed to engine-bay and cabin temperature extremes
- Industrial control equipment and inverters operating in harsh thermal environments
- Consumer electronics and LED lighting, where compact enclosures produce elevated operating temperatures
- Component incoming inspection and supplier quality audits, where temperature coefficient data verifies batch consistency and process control
FAQ
What is a typical temperature coefficient value for a zinc oxide varistor?
Most general-purpose ZnO varistors exhibit a temperature coefficient with an absolute value of approximately −0.05% to −0.1%/°C within the category temperature range. The exact limit is defined by the manufacturer's detail specification or the applicable procurement standard, and automotive or industrial grades often require tighter control.
Why does varistor voltage decrease as temperature increases?
The nonlinear conduction of a ZnO varistor is governed by Schottky barriers at the grain boundaries. Higher temperatures thermally activate charge carriers, lowering the effective barrier height and reducing the voltage required to drive the specified test current, which produces a negative temperature coefficient.
What test current is used for varistor voltage measurement in temperature coefficient testing?
The standard test current is 1 mA DC for most disc and block varistors, while 0.1 mA DC is commonly used for small SMD chip varistors. The current is applied as a short pulse to prevent self-heating from affecting the result.
Which temperature points are evaluated?
Testing normally compares the varistor voltage at the reference temperature (20°C or 25°C) with values measured at the lower and upper category temperatures, commonly −40°C and +85°C for commercial products, with extended ranges such as +125°C or higher for automotive applications.
Why is leakage current also measured at high temperature?
Leakage current rises significantly with temperature. If leakage at the maximum continuous operating voltage becomes excessive at high ambient temperature, it can cause resistive self-heating, accelerate aging, and in extreme cases lead to thermal runaway. High-temperature leakage testing therefore complements the temperature coefficient measurement as a reliability screen.
Can temperature coefficient testing be combined with other qualification tests?
Yes. Accredited third-party laboratories commonly combine temperature coefficient measurements with surge current withstand, energy handling, climatic endurance, and life testing as part of a full varistor qualification program aligned with IEC 61051, GB/T 10193, and related standards.